chapter 3 physical transformation processes of forest ......2.1 natural drying natural drying is...

CHAPTER 3

Physical Transformation Processes of Forest Biomass: Chipping, Bundling, Drying and Control of Particle Size Distribution

Daniel J. Vega-Nieva & Luis OrtizForestry School, University of Vigo, Campus A Xunqueira, 36005 Pontevedra, Spain.

Abstract

Forest biomass is characterized by physical properties such as high moisture con-tent, low density and heterogeneous particle size, which condition its direct use for combustion. Hence, processes of physical transformation are previously required to obtain products of higher energetic value, reducing transport costs and allowing for homogeneous boiler feeding. The biomass harvesting processes, as well as the logis-tic options for chipping and bundling, are described in this chapter, which offers a summary of the costs of such operations measured in forest biomass logistic studies in Spain. Studies on natural versus controlled drying of chips and bundles are also discussed, and general recommendations for the management of moisture content and the prevention of self-ignition risk during storage are given. Finally, experiments to date involving the screening and size reduction of forest biomass are summarized.

Keywords: Forest biomass, chipping, bundling, drying, screening, size reduction.

1 Forest Biomass Harvesting and Transportation

There are two main types of forest biomass sources: residual forestry biomass and energy crops. The difference resides in the harvesting and transportation costs due to their logistic chains, and they will be analysed separately.

1.1 Harvesting and transportation costs of forest residual biomass

Several experiments in forest residue collection for bioenergy compare the pro-ductivity and costs of field chipping, forest track chipping and bundling strategies on different coniferous and deciduous forest harvesting [1,2].

www.witpress.com, ISSN 1755-8336 (on-line) WIT Transactions on State of the Art in Science and Engineering, Vol 85, © 2015 WIT Press

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38 Biomass Pelletization: standards and Production

• Residue chipping in the forest track involves accumulating residues with a for-warder, and allowing the moisture content of residues to decrease by natural drying at the forest track piles. According to the Spanish IDAE [3] guidelines, this is the most economical system for large forests. It is, moreover, the option favoured by European Nordic countries with experience in biomass residue collection [4].

• The bundling system, on the other hand, is limited by a high machinery cost of 98 €/h [5] and the need for posterior chipping of residues; its use is on the decline with respect to chipping systems.

A series of experiments in collection and treatment of forest residues (Pinus pinaster and Eucalyptus globulus) were conducted in Galicia, NW Spain, by Sanz and Piñeiro [1] to compare the productivity of diverse collection systems. Three different residue situations were considered (Fig. 1):

• Situation 1: Residues scattered on the field.• Situation 2: Residues grouped in small piles on the field.• Situation 3: Residues grouped in large piles at the forest track.

The piling of residues (situations 2 and 3) reduced biomass moisture content and generally decreased collection costs as compared to situation 1, where resi-dues were not grouped previous to collection.

The operation productivity, residue moisture content (MC) and residue collec-tion costs (in 2003) obtained in this study are summarized in Table 1 for the four different scenarios (forwarding distance = 300 m) considered in the study (Fig. 2):

• SC. 1.a: Chipharvester Bruks 803 CT on Valmet 892 forwarder.• SC. 2.a: Pezzolato PHT 1200 chipper on Timberjack 1210 B forwarder.• SC. 3.a: Fiberpac 370 B Bundler on Timberjack 1210 B forwarder.• SC. 4 a: Balapress Bundler.

Figure 1: Types of biomass collection logistics studied by Sanz and Piñeiro [1] in NW Spain. From left to right: situation 1: residues scattered on the field; situation 2: residues grouped in small piles on the field; situation 3: residues grouped in large piles at the forest track.


Physical transFormation Processes oF Forest Biomass 39

Figure 2: Mobile chippers and bundlers utilized in the Sanz and Piñeiro [1] study in NW Spain.

Table 1: Summary of Sanz and Piñeiro [1] biomass collection experiences in Galicia, NW Spain.

Scenario Species MC (%)

Operation productivity (green tonnes/h)

Residue collection costs (€/green tonne)

SC.1a Pinus situation 2 35 10.7 8.9Pinus situation 3 31 14 6.8Eucalyptus situation 1 47 9 10.6Eucalyptus situation 3 26 12.6 7.6

SC.2a Pinus situation 3 31 13.6 7.6SC.3a Pinus situation 2 35 9.6 8.3

Pinus situation 3 31 9.3 8.6Eucalyptus situation 1 47 9.9 8Eucalyptus situation 3 26 13.1 6.1

SC. 4a Pinus situation 3 (average production)

31 3.4 28.8

Pinus situation 3 (maximum production)

31 4.9 20

These results were updated to prices of 2007 by Pedras [2] to include an estima-tion of transport cost while assuming an optimal organization of biomass collec-tion and treatment and a transport distance of 100 km. In this case, the cost for the biomass plant would range between 23 and 28 €/green tonne for the field chipping



with a chip harvester, and between 27 and 32.8 €/green tonne for the Fiberpac col-lection and plant chipping (Table 2). These costs include neither industrial profit nor indirect costs.

The Polytechnic University of Madrid, in cooperation with CESEFOR, conducted several studies of biomass residue collection in different types of harvests of various Pinus, Quercus and Populus stands [4,6]. Table 3 summarizes the total costs obtained in the cases studied (SC. 1b: whole tree harvesting and chipping at forest; SC. 2b: residue collection and chipping at forest; SC. 3b: mobile chipping at forest; SC. 4b: bundling and chipping at plant), including industrial benefit and indirect costs.

1.2 Harvesting and transportation costs of woody energy crops

Woody energy crops are generally established at higher densities than conven-tional forestry plantations, and can be managed in short rotation coppice systems, with rotations of 2–3 years generally for willow (Salix sp.) and 2–5 years for pop-lar [7]. Poplar energy crops were established commercially in Italy, where 5,000 hectares are managed for chip production, and commercial willow energy crops have been established in Sweden and the UK, based on specifically bred clones for biomass production [8].

Table 2: Total costs of the highest productivity scenarios from the CIS-Madera experiences in Galicia [2].

Scenario Costs of biomass at plant (€/green tonne)

Residue collection

Transport (100 km)

Chipping at plant

Total cost

SC 1.a. 11.9–14.5 11–13.5 – 23–28SC 3.a. 13.5–16.5 10–12 3.5–4.5 27–33

Table 3: Summary of costs obtained in forest biomass residue utilization experiences in Castilla-Leon, Spain.

Scenario Species MC (%) Costs (€/green tonne)

Distance to plant (km)

SC. 1b Quercus pyrenaica 42 54 100Pinus silvestris 56.2 34.2 40

SC. 2b Populus 45.5 40.9 100SC. 3b Pinus pinaster 35 27.4 40

SC.4b Populus 50 38 50Pinus 50 33–47 140–150



Information on current research on varieties selection and management optimization of woody energy crops can be found in the literature [7,9–11].

In Spain, the studies of Ciria et al. [12–14] and San Miguel and Montota [15] have determined the yield potential of high-density poplar energy crop planta-tions for biomass productions, with yields of 10–18 dry matter tonne/ha in rota-tions of 3–6 years at densities of 5,000–10,000 stems/ha and higher. A recent study [16] analysed the economic feasibility of poplar energy crop plantations for bioenergy production, considering a life cycle of 16 years in three harvests at 5-year coppice intervals, comparing whole tree harvesting and field chipping systems.

2 Forest Biomass Drying

Woody biomass commonly has high moisture content after harvest, which implies limitations on its energetic utilization that includes:

• Increase of harvest and transport costs.• Decrease of the efficiency in the physical transformation stages.• Low efficiencies in the thermochemical conversion processes.• Increase of CO2 and NOx emissions during combustion.• Unstable combustion.• Limitations or avoidance of its use as a fuel.

To overcome these limitations, moisture content in forest biomass must be decreased up to values of 20%–30% by natural or artificial drying.

Table 4: Total biomass costs at plant for Populus energy crop [16].

Yield, DM tonne/ha/year

System* Costs (€/green tonne)

Production and harvesting

Transport Chipping Total costs

20 W. tree 4.8 2.93 2.26 9.9Chips 5.2 4.24 9.4

15 W. tree 6.4 2.93 2.26 11.5Chips 7.0 4.24 11.2

13.5 W. tree 7.3 2.93 2.26 12.4Chips 7.7 4.24 11.9

9 W. tree 10.9 2.93 2.26 16.0Chips 11.6 4.24 15.8

*W. tree: whole tree harvesting (MC = 50%, transport at 20%); chips: tree chipping (MC = 50%).



2.1 Natural drying

Natural drying is performed when the climatic conditions allow for the loss of moisture controlled by the environmental temperature and relative humidity con-ditions, under the absence of precipitation. In the case of forest biomass residues from harvest, there are two possibilities for natural drying: to conduct the drying on the field directly after harvest or to conduct the drying after chipping. In cold climate countries, for instance in Northern Europe, harvest residues are often piled in the field until attaining a field stockage of biomass that will dry under environ-mental conditions in the field (Fig. 3).

2.1.1 Thermogenesis processDuring the storage of woody biomass, thermogenesis may occur due to the living cells of the parenchyma, the biological activity of microbial bacteria and fungi, in conjunction with the oxidation and hydrolysis of cellulose that result in energetic losses of the woody fuels during storage. The initial release of heat is mainly produced by the respiration of the live cells of the parenchyma and the bacterial growth. At temperatures above 40°C, the cells that were alive die gradually; the evolution of heat is mainly caused by fungal respiration, yet above 45°C–50°C the chemical reactions releasing heat become increasingly important [18].

The temperature reached in piles of woody residues also depends on the envi-ronmental temperature, precipitations, size and compaction of the pile, and the amount and distribution of bark and fine material. In the central area of biomass piles, temperature rises quickly in the first weeks of storage, after which it stabi-lizes and eventually decreases progressively [17].

Under certain conditions, self-ignition of the piles may occur. This dangerous phenomenon is more likely in big piles, starting in the cavities containing fine ele-ments and bark. The self-combustion risk increases when residues contain large amounts of bark, or when ice layers or fine material accumulations obstruct the cavities, thereby preventing the exchange of heat in the atmosphere.

2.1.2 Drying dynamics in biomass pilesBiomass pile dynamics involve the so-called “chimney effect” (Fig. 4): As the air penetrates inside, the piles form the surface, decreasing the temperature of the

Figure 3: Open air natural drying of wood chips at the CIEMAT Renewable Energy Centre in Soria, central Spain [17,18].



biomass residues. This current of air heats up and acquires the moisture of the residues as it advances towards the pile centre, resulting in a flow of warm and wet air from the centre of the piles to the top. The water consequently flows vertically and accumulates at the pile top, where a strong increase in temperature and vapour exchange to the atmosphere occurs. The increase in temperature is favoured by the accumulation of microorganisms transported by the air flow, which accelerates the growth of the microbiological population and, consequently, the risk of thermo-genesis. This vapour loss gives an impression of smoke coming out from the top of piles, hence the name of this phenomenon, chimney effect. In a biomass pile, the following three zones can be clearly distinguished (Fig. 5):

Figure 4: Air flow inside biomass piles.

Figure 5: Zones that can be differentiated within a forest biomass pile.



• A surface area of a varying depth susceptible to important modifications in the moisture content, due to the direct influence of environmental conditions.

• An inside area where a slow and sustained drying occurs, with no direct influ-ence by the environmental conditions. Moisture loss is instead driven by an air flow that transports water progressively, and self-eating processes that acceler-ate the drying until moisture content is stabilized.

• A top area where the water transported from the centre and the water from the environment accumulate, resulting in the highest moisture content of the bio-mass piles.

2.1.3 Experiences with natural air drying of forest chipsThe study by Ortiz [17] on the natural air drying of forest wood chips and other forest residues arrived at the following practical conclusions for managing residual biomass piles to be dried:

• The drying season is the factor with the highest influence on the process. Under the climate conditions of central Spain, summer drying is the most favourable. In this season, 2–3 weeks sufficed to reach moisture contents of 20%–30% w.b., whereas in spring 10–15 weeks were needed to reach those values.

• When chips are generated in autumn, even after drying biomass in piles previ-ous to chipping, values of 20%–30% were not reached; a balanced moisture content of 35% was attained within 1–2 weeks.

• Large size chips are capable of faster drying than smaller particle size chips, because the ventilation of air is better between the cavities of bigger chips.

• During storage, microbial activity results in monthly dry matter losses of around 1%. This value is significantly lower in the case of air drying previous to chipping.

• This procedure makes it possible to harvest 60%–75% of relatively dry chips, even in short time periods.

It is important to establish a management of the piles that allows a partial dehu-midification without reaching limits that result in important losses of energy and can even be hazardous. Ortiz [17] established consequently the following storage recommendations:

• Piling of chips with conical shape of the piles in winter, with slopes 1:1.• Piling of chips with trapezoidal shape of the piles in summer.• Pile sizes should not be larger than 30–50 m3.• Make piles by gravity, avoiding compaction.• Avoid compaction of piles by machinery.• Prevent as much as possible the presence of fines on the pile surface.• When vapour is seen coming out of the chip piles, cut the head of the pile.• When temperatures higher than 60°C are observed, turn over the piles.• During the dry season, extend the chips in layers of 30 cm and turn over the chips.



2.1.4 Experiences with natural drying of forest bundles under open air, roof covered and controlled temperatures

The study of Dopazo et al. [19] monitored the moisture loss of Eucalyptus camal-dulensis bundles from the Spanish Energy and Pulp Company – ENCE – at the Faculty of Forest Engineering, University of Vigo (Fig. 6).

Weight loss was monitored for 40 days for bundles drying under open air, roof cover and under controlled moisture and temperature conditions in two drying chambers with controlled temperature and relative humidity conditions. Con-trolled drying chamber 1 simulated mild summer conditions of NW Spain (tem-perature: 25°C, relative humidity: 55%), whereas chamber 2 monitored the weight loss under warm summer conditions (temperature: 30°C, relative humidity: 40%).

Sharper moisture loss was observed under warm summer conditions. A mois-ture equilibrium was reached after 1–2 weeks of drying, both in the open air and under the simulated warm summer conditions, to dry camera bundles (Fig. 6). Bundles drying indoor followed the mild summer conditions drying curve (Fig. 6).

2.2 Artificial drying

When it is not possible to decrease the moisture content of the biomass with natu-ral air drying, or when low moisture content values are required (e.g.


• Rotatory driers are generally utilized when very wet or coarse materials are utilized. Even though they involve the same components as pneumatic driers, in this equipment the channel of circulation is a cylinder of varying length and section. It rotates at a variable speed, inducing close contact between the bio-mass fuels and the drying flux. Together, the inner slope and the rotation pro-duce an advance of the fuels at a controllable speed. To decrease the length of the rotatory driers, they may be constructed with a single, double or triple pass (Fig. 7); the fuels thus pass through the dryer at alternating routes.

Table 5 summarizes the results obtained from the artificial drying of pine, Euca-lyptus and Goarse (Ulex spp.) in the experimental rotatory dryer (IER-CIEMAT) shown in Fig. 8.

2.2.1 Artificial drying curves for forest residues with varying temperatureOrtiz et al. [20] performed a study of controlled artificial drying of thick branches (2.5–5 cm diameter) of Pinus pinaster and Eucalyptus globulus, with controlled drying at 30°C, 60°C and 105°C in a furnace for 200 h. A sharp decrease in mois-ture was observed for 60°C and 105°C drying, with equilibrium values reaching within the first 24 and 36 h, respectively, for the two species. At 30°C, which might be representative of the air drying under summer conditions of a thin layer of forest residues, moisture in equilibrium was reached after 100 h, this being the characteristic drying time of medium-size fuels such as branches [21].

3 Size reduction

The grinding and/or milling of forest residues becomes necessary when utilizing biomass in conjunction with equipment designed to utilize products finer than chips. It is, moreover, a pre-requisite for the fabrication of pellets, where a smaller particle size and higher product homogeneity are required. This physical trans-formation is achieved after the reduction of moisture content in order to obtain smaller particles.

Energy consumption with respect to size reduction depends on the size of par-ticles. As illustrated in Fig. 9, higher consumptions imply more finely sized fuels.

Figure 7: Examples of rotatory dryer designs.



Table 5: Parameters in the controlled drying of several forest species in an experimental rotatory dryer at the CIEMAT Renewable Energy Centre in Spain [18].

Parameter Pinus Eucalyptus Ulex

Capacity kg/h 1,280 1,292 1,520 Initial moisture % w.b. 38.8 23.7 59.6 Final moisture % w.b. 6.3 6.6 4.2 Environmental temperature °C 26.4 19.4 14.2 Relative humidity environment % 34.7 45.5 64 Temperature of furnace °C 438 458 550 Gas temperature entrance °C 385 410 493 Gas temperature exit °C 66.8 68.6 64.3 Moisture gas exit % 12.7 10.67 12.1 Energy consumption kW 40 63 64 Trommel speed rpm 9 9 10 Gases kg/h 7,004 3,369 9,974 Gas flow Nm3 5,448 2,620 7,758 Evaporated water kg/h 443.9 236.5 879 Air/wood (d.b.) kg/kg 8.94 3.42 16.2 Dry air/wood (d.g.) kg/kg 0.27 0.2 0.29

Dry air/evaporated water kg/kg 15.78 14.24 11.35 Consumption of fuel (pellet A1) kg/h 203 183 187

Figure 8: Experimental rotatory dryer (IER-CIEMAT) [18].

Once the biomass has been reduced, it is necessary to ensure a homogeneous particle size for automatic feeding systems of pellet mills or chip boilers. In addi-tion, fine elements increase ash content and may result in increased ash slagging. The European standard EN 15149-1 [23] establishes limits for both coarse and fine



Figure 9: Energy consumption in terms of size reduction at the forest biomass densification pilot plant (shown in Fig. 10) of the Faculty of Forest Engineering, University of Vigo, testing varying fuel feeding amount and particle sizes [18].

Figure 10: Details of the forest biomass densification pilot plant of the Faculty of Forest Engineering, University of Vigo, with a detail of the patented milling hammer system [22]. More information of the pilot plant can be found at: http://lortiz.webs.uvigo.es/?page_id=20



materials, and categorizes chip sizes from P16 to P45 (large chips). Recent ISO standards also include categories for larger chips such as P90 (extra coarse chip). This is the reason behind the growing number of companies performing chip screening at biomass logistic centres (Figs. 11–13) to deliver a homogeneous qual-ity fuel of normalized physical properties.

Figure 11: Screening of residual forest biomass at the Forest biomass pilot plant of the Faculty of Forest Engineering, University of Vigo [18].

Figure 12: Biomass logistic centre in Palas de Reis, NW Spain [24]. Top figure: woody biomass chipper and debarker. Bottom figure: a screen with variable opening size is utilized for controlling the granulometric dis-tribution of wood chips to a P31 or P45 size category. Fine fractions of


References

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[2] Pedras, F., Xestión e mecanización da biomasa forestal. Exp. en Galicia, 2008. [3] IDAE, Biomasa: Cultivos Energéticos. IDAE: Madrid, Spain, 2007. [4] Tolosana, E., Ambrosio, Y., Laina, R., Martinez, R. & Pinillos, F., Ren-

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[7] Kauter, D., Lewandowski, I. & Claupein, W., Quantity and quality of har-vestable biomass from Populus short rotation coppice for solid fuel use – a review of the physiological basis and management influences. Biomass and Bioenergy, 24, pp. 411–427, 2003.

[8] Larsson, S., Bullard, M.J., Christian, D.G., Knight, J.D., Lainsbury, M.A. & Parker, S.R. Commercial varieties from the Swedish willow breeding

Figure 13: Biomass logistic center in Orense, NW Spain [24]. Woody biomass is chipped after storage and dried under open air conditions. Chipped wood and bark are screened for the production of wood chips of nor-malized sizes P16–P90.



programme. Biomass Energy Crop. II, University of York, York, UK, 18–21 December 2001, Association of Applied Biologists, 2001, pp. 193–198.

[9] Sixto, H., Hernandez, M.J., Barrio, M., Carrasco, J. & Cañellas, I., Plantacio-nes del género Populus para la producción de biomasa con fines energéticos: revision. Sist Recur For, 16, pp. 277–294, 2007.

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[12] Ciria, M.P., Mazon, P., Carrasco, J.E. & Fernandez, J., Effect of rotation age on the productivity of poplar grown at high plantation density. 8th Eur. Bio-mass Conf., 1995, pp. 489–494.

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[14] Ciria, M.P., La Producción de Chopo con fines energéticos en España. I Congr. Int. Bioenergía. La Bioenergía, una oportunidad, una necesidad, 2006.

[15] San Miguel, A. & Montota, J.M., Resultados de los primeros 5 años de pro-ducción de tallares de chopo en rotación corta (2–5 años). An INIA, Ser For, 8, pp. 73–91, 1984.

[16] Gasol, C.M., Martínez, S., Rigola, M., Rieradevall, J., Anton, A, Carrasco, J., et al., Feasibility assessment of poplar bioenergy systems in the Southern Europe. Renew Sustain Energy Rev, 13, pp. 801–812, 2009.

[17] Ortiz, L., Air Drying of Forest Residual Biomass. Polytechnic School of Madrid: Madrid, 1989.

[18] Ortiz, L., Aprovechamiento Energetico de la Biomasa Forestal, 1996.[19] Dopazo, R., Vega-Nieva, D. & Ortiz, L., Drying of Eucalyptus bundles under

open air, roof-covered and under simulated mild and warm summer conditions, 2011.

[20] Ortiz, L., Tejada, A., Vazquez, A. & Piñeiro, G., Aprovechamiento de la Bio-masa Forestal producida por la Cadena Monte-Industria. Parte III: Produc-ción de elementos densificados. CIS-Madera, 10, pp. 38–75, 2003.

[21] Anderson, H.E., Aids to determining fuel models for estimating fire behav-ior, 1982.

[22] Ortiz, L., La biomasa como fuente de energía renovable, 2006.[23] EN15149-1, Solid biofuels. Determination of particle size distribution. Part

1: Oscillating screen method using sieve apertures of 1 mm and above, 2010.[24] Vega-Nieva, D., Alvarez, C. & Ortiz, L., Results of new laboratory methods

and slagging classification systems for the prediction and quantification of ash slagging in woody and herbaceous biomass fuels. Cent. Eur. Biomass Conf., 2014.


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